Post-Apocalypse

Dr. Subramanian managed to escape the chaos that overcame Cell Block D. After the zombified residents were put down, he tended to the injured survivors and later examined Charlie's corpse. He discusses with Rick Grimes, Tyreese, Hershel, Sasha, Bob Stookey, and Daryl about the illness affecting the prison group, stating that it's a type of flu that spreads quickly and proceeds to explain how.

Dr. Subramanian is seen treating a flu-stricken survivor with an amputated arm from the cell block attack. The man succumbed to the illness and is put down with a knife to the head. When Sasha falls ill, she starts looking for Caleb, until she finds out that he also has come down with the sickness. Since Caleb is no longer able to treat people, Hershel volunteers to fill his role. When Hershel is comforting Caleb, he accidentally coughs blood on Hershel's face and tells him he shouldn't be in there with him.

Dr. Subramanian creates IVs for the infected survivors, despite his rapidly deteriorating condition. He reveals to Hershel that he brought several shotguns with him for the inevitable. He shrugs off Hershel's attempts to persuade him that things aren't bad. Later, during the chaos after several survivors reanimate, Hershel runs to his cell for help, only to find that he has died and reanimated. Hershel reluctantly breaks his arm, and stabs him through the eye. After the chaos dies down, he comes back to Caleb's cell and closes his other eye before opening his Bible and reading through it. However, Hershel breaks down and weeps over the loss of his friend and the other prison residents.

Death

Killed By

The Flu (Alive)

Caleb was one of those who became infected with the flu. As time passed, his health gradually deteriorated despite Hershel's best efforts to keep him well. Caleb ultimately succumbed to the flu and died while locked in his cell.

When Hershel goes to his cell for help in getting the guns, a reanimated Caleb attacks him through the cell bars. Hershel breaks his arm, then stabs a knife through Caleb's left eye socket, putting him down. Later, after the chaos has ended, Hershel goes to his cell, strokes his head, and begins to cry.

Relationships

Dr. Subramanian is seen conversing with Rick, Daryl, Hershel, and Bob about the unknown flu and explains how Charlie likely died, noticing blood leaking from his eyes and nose and a lack of bites. Rick then mentions Patrick's similar condition. When Caleb is sick, Hershel gives him some herb tea, hoping that it will help with the symptoms. They briefly talk, showing that they have a good relationship with one another.

It is implied, due to the fact that both are doctors, they are sympathetic towards each another, mutually understanding the difficulties of the job. Caleb repeatedly tells Hershel that he shouldn't be in the cell block with them, conveying that he cares about him. Hershel apparently believes Caleb to be a good man, which is evident through their conversations and how Hershel repeatedly emphasizes that Caleb would do the same thing if "he were in his shoes." Hershel shows concern over Caleb's rapidly deteriorating condition. When Caleb becomes pessimistic over the flu and his symptoms, Hershel comforts him and tells him that they're "not that bad off." Caleb disagrees and gives Hershel the advice to make sure everyone is locked in, and shows him the shotguns he brought into his cell.

Later, when Hershel returns to Caleb's cell and discovers him as a zombie, he stabs him in the head.

After Glenn is stabilized and all has calmed down, Hershel comes back to the doctor's cell and closes the dead man's eye. He pulls out his Bible and begins to read, weeping over the loss of his friend.